Optoelectronic microfluidic devices (e.g., optoelectronic tweezers (OET) devices) utilize optically induced dielectrophoresis (DEP) to manipulate objects (e.g., cells, particles, or the like) in a liquid medium. FIGS. 1A and 1B illustrate an example of a simple OET device 100 for manipulating objects 108 in a liquid medium 106 in a chamber 104, which can be between an upper electrode 112, sidewalls 114, photoconductive material 116, and a lower electrode 124. As shown, a power source 126 can be applied to the upper electrode 112 and the lower electrode 124. FIG. 1C shows a simplified equivalent circuit in which the impedance of the medium 106 in the chamber 104 is represented by resistor 142 and the impedance of the photoconductive material 116 is represented by the resistor 144.
Photoconductive material 116 is substantially resistive unless illuminated by light. While not illuminated, the impedance of the photoconductive material 116 (and thus the resistor 144 in the equivalent circuit of FIG. 1C) is greater than the impedance of the medium 106 (and thus the resistor 142 in FIG. 1C). Most of the voltage drop from the power applied to the electrodes 112, 124 is thus across the photoconductive material 116 (and thus resistor 144 in the equivalent circuit of FIG. 1C) rather than across the medium 106 (and thus resistor 142 in the equivalent circuit of FIG. 1C).
A virtual electrode 132 can be created at a region 134 of the photoconductive material 116 by illuminating the region 134 with light 136. When illuminated with light 136, the photoconductive material 116 becomes electrically conductive, and the impedance of the photoconductive material 116 at the illuminated region 134 drops significantly. The illuminated impedance of the photoconductive material 116 (and thus the resistor 144 in the equivalent circuit of FIG. 1C) at the illuminated region 134 can thus be significantly reduced, for example, to less than the impedance of the medium 106. At the illuminated region 134, most of the voltage drop is now across the medium 106 (resistor 142 in FIG. 1C) rather than the photoconductive material 116 (resistor 144 in FIG. 1C). The result is a non-uniform electrical field in the medium 106 generally from the illuminated region 134 to a corresponding region on the upper electrode 112. The non-uniform electrical field can result in a DEP force on a nearby object 108 in the medium 106.
Virtual electrodes like virtual electrode 132 can be selectively created and moved in any desired pattern or patterns by illuminating the photoconductive material 116 with different and moving patterns of light. Objects 108 in the medium 106 can thus be selectively manipulated (e.g., moved) in the medium 106.
Generally speaking, the unilluminated impedance of the photoconductive material 116 must be greater than the impedance of the medium 106, and the illuminated impedance of the photoconductive material 116 must be less than the impedance of the medium 106. As can be seen, the lower the impedance of the medium 106, the lower the required illuminated impedance of the photoconductive material 116. Due to such factors as the natural characteristics of typical photoconductive materials and a limit to the intensity of the light 136 that can, as a practical matter, be directed onto a region 134 of the photoconductive material 116, there is a lower limit to the illuminated impedance that can, as a practical matter, be achieved. It can thus be difficult to use a relatively low impedance medium 106 in an OET device like the OET device 100 of FIGS. 1A and 1B.
U.S. Pat. No. 7,956,339 addresses the foregoing by using phototransistors in a layer like the photoconductive material 116 of FIGS. 1A and 1B selectively to establish, in response to light like light 136, low impedance localized electrical connections from the chamber 104 to the lower electrode 124. The impedance of an illuminated phototransistor can be less than the illuminated impedance of the photoconductive material 116, and an OET device configured with phototransistors can thus be utilized with a lower impedance medium 106 than the OET device of FIGS. 1A and 1B. Phototransistors, however, do not provide an efficient solution to the above-discussed short comings of prior art OET devices. For example, in phototransistors, the light absorption and electrical amplification for impedance modulation are typically coupled and thus constrained in independent optimization of both.
Embodiments of the present invention address the foregoing problems and/or other problems in prior art OET devices as well as provide other advantages.